Electrical sound-detecting apparatus



, f 1,446,544* P. w. BRIDGMAN v ELECTRIC-AL VSQUND DETECTIN. APPARATUS Feb. 27, 1923;

F11ed June 25, 1919` K sweets-sheet 1 Fehn-1923; l Y 1,446,544

' I P. W. BRIDGMAN ELECTRICAL SOUND DETECTING APPARATUS HW/ENTOR.v

Patented Feb. 27, 1923.

' UNETE'D STATES PATENT OFFlCE.

PERCY W. BRIDGMAN, OF CAMBRIDGE, MASSACHUSETTS.

-ELECTRICAL SOUND-DETECTING APPARATUS.

Application filed June 25, 1919. Serial No. 306,69051.

Ations or jars.

Referring to the drawings, Figs.-1, 2 and 3 are an end elevation, a side elevation and a vertical section of ainicrophone embodying the invention-` Fig. 4 is a side elevation showing the microphonemounted on a vibrating diaphragm. Figs.l 5, 6 and T are an endelevation, a side elevation and a vertical section showing :a modification. -Fig` 8 is a wiring diagram showing the receiver connection to the microphone. Fig. 9 is a wiring diagram showing av number of microphones connected in parallel. Fig. lO'is a Wiring diagram showing microphones connected'in series.

The invention is illustrated as embodied lin a double cell inertia microphone. Referring particularly to the microphone as illustrated in Figs. 1 to 4, the microphone 1 has two carbon granu-le containing cells 2 and 43, arranged end .to end.

two end contact. plates 5 and 6 of the usual hard carbon. The plates 5 and 6 are soldered -in -the usual way to metal backing embedded in the' sound receiving diaphragm- 13. The diaphragm 13 is illustrated as a soft flexible -rubberdiaphragm, which is the material. ordinarily employe-d for submarine sound receivers 'f The metal plate 8 at the other end of the i cell2 is connected by means of the threaded stud 20 to a metal supporting plate 21.

The carbon granule containing space and in Electrical The object of the invention is to make g The carbonY granules '4 in cell 2 are contained between the plates 5, 6, 7 and 8 are surrounded by a casing 22 made of a shortpiece of soft india rubber tubing. Y

Thesecond microphone cell 3 is of similar construction. The carbon granules 24 are contained between carbon plates 25 and 26,.

which are soldered to metal plates 27 and 28. The metal plate 27 is mounted on an end supporting plate 29 by means of the stud 30 and nut 31. The center metal plate 28 is mounted on a metal plate 32 by means of a threaded stud 33. The cell is enclosed in a soft rubber casing 22.

The center plates are cemented to a sheet of paper insulation 34 placed between them.

AThe end plates 11 and 29 are rigidly yoked together by asheet metal casing 40." Thin soft rubber shims 41 may be interposed between the edges of the plates 21 and 32 and' the inside of the metal casing 40 to steady. the plates 21 and 32. In the vmicrophone illustrated in Figs. 1 to 4, the end plates act as the agitating plates while the center .plates act as the vibration resisting ,or inertia plates. When the diaphragm 13 is thrown into vibration by incident sound waves, the sound vibrations are transmitted to the end contact plates 5 and 25 of the -microphone cells. The central plates 6 and 26, are rigidly connected together through thel central plates 21 and 32 and form with these plates an inertia member which tends to remain stationary andby its inertia resists the vibrations impressed on it through the carbon granules and the flexible soft rubber houslngs 22. As the sound vibrations are comparatively rapid, the inertia of the center plates is sufficient to affect a noticeable compression and release of the carbon granules in the cells. When the diaphragm 13 moves to the left under the in- 2and 3 are oppositely and equally affected,

and as hereinafter explained, by connecting a telephone receiver to a point between the two cells, this differential effect is utilized.

The contact plates 5, 6, 25 and 26 are connected through-lead wires 50, 51, 52 and 53 togthe external circuits.

In Figs. 5, Gland 7 is shown a modification in which the center plates are the agitated plates and the end plates are the inertia plates. dThe carbon granule containing cells in 62 and' 63 are arranged end to end. The end carbon plates 64 and 65 are soldered to small brass plates 66 and 67 which in turn are soldered to end supporting plates 68 and 69. These-end plates 68 and 69 are rigidly connected together by struts 70 of insulating material. The end plates with their supporting members form the inertia. element of the microphone. The` center contact plates 71 and-72 are` soldered to brass plates 73 and 74 which are in turn secured to ay supporting metal plate 75. The metal plate 75 is connected through-the spacer screws 76 to the plate 77 which in turn is rigidly mounted on the post 78 which is carried b v the diaphram 79. The microphone cellsare enclosed in soft rubber casings 80 which permit the inertia member to float and permit the proper pressure and release of the 'carbon granules in the cells under the vibrations of the agitating member. Soft gum india rubber tubing has been found particularly successful for this use as it transmits of itself a minimum vibration from the agitat-ing' to the inertia members, and it also tends to damp out any free vibrations in the inertia member.

In the form of microphone shown in Figs. 5, 6 and 7, the two center plates 71 and 72 are not insulated from each other, but are in electrical connection with the supporting plate' 75 from which the lead wire 90 is tapped off. The lead wiresv 91 and 92 are connected with the end plates 68 and 69 respectively.

When a single microphone is used, it is connected as shown in Fig. 8. The two microphone cells 'A and B are connected through lead wires C and D to battery E. A branch circuit F is tapped ott' between the microphone cells A and B and contains the telephone receiver (l. The other end of this branch circuit F is connected to the. battery E. For reasons of symmetry it is preferred to connect this branch circuit to the lniddle of the battery. This. however is not necessary as it may be connected to either end of the battery .by a connection to one of the leads C or D. A condenser H. is preferably interposed in the branch circuit 1` to protect the receiver from the battery current. However, the presence of the condenser H in this circuit. is not a necessity.

The to-and-fro if'ibrations of the diaphragm upon which the microphone is mounted cause the resistant-es of the microphone cells A and B to be oppositely affected, one being increased whilethe otheris decreased. The resistance variations are in opposite phase to each other and combine to shift with respect to the battery the potential of the point in the battery circuit between the two cells where thel branch circuit F is tapped off. These out-of-phase components of the resistance variation of the microphone cells therefore combine to send current through the receiver G in the branch circuit. It istobe particularly noted that it is only the out-of-phase components of the resistance variations o-f the cells which cause any current flow through the telephone receiver G and therefore the receiver is responsive only to the to-and-fro vibrations of the microphone supporting diaphragm. Any motion or conditions which affect -the micro-phone as a-whole do not send a current through receiver G. For example, any mo- -tion of the microphoneI supporting diaphragm which takes place in the plane of such a. diaphragm tends to cause an equal increase or decrease of the resistance ofthe twomicrophone c'ells,lcausing in-phase resistance varia-tions which send current around the battery circuit as a whole but do not cause a flow of current through the branch circuit F. This characteristic of the apparatus renders it of particular value for use on shipboard or insubmarine listening devices carried upon ships or floated objects. It is found that when microphones of this type are used on submarine sound detect-ing devices, they are little, if any, affected by the rise and fall of the ship inthe waves. The receiver G is also substantially independent of electrical variations in the battery circuit as a whole. For example, a loose contact or. other cause of variable potential impressed on the microphone does not cause a differential effect which sends current through the receiver.

1V hen it is desired, as forexample in some types of submarine listening apparatus, a number of the microphones may be connected-together. In Fig. 9 is shown a parallel arrangement of microphones in which the microphones are connected in parallel to the battery leads C and D and the telephone receiver circuit F is tapped off from the middle points of the several microphones. lVhen a single microphone is used, as shown in Fig. 8 or the parallel arrangement is used as shown in Fig. 9, the two center contact plates of the microphones may be electrically connected together in` the microphone as shown in Fig. 7. When the microphones are put in a series connection, the central contact plates are insulated from each other as shown inv Fig. 3. A series connection of microphones is' shown in Fig. 10, in which the battery current goes through all the microphone cells in series, and the branch circuit F which contains the telephone receiver, is tapped off at a point midway between the several microphone cells.

From the foregoing description it will be seen that the useful effect in the telephone receiver' is that produced by the equal and .erate properly the two cells should be matched so that these resistance variations are as near equal as possible for each of the two cells. To attain this the yielding connections between the plates of the two cells should be the same. It has been foundwfrom a long series of experiments that the soft rubber casings such as are shown at 22 and 8O in Figs. l to 7 are the most satisfactory material for this purpose. Such soft rubber casings preserve the proper spacing between the plates of the two cells and' at the same time form a readily `vielding connection between .them. They permit substantially the same yielding between the contact plates of the two cells with a consequent matching of the resistance variations ofbthe respective cells. This feature is emphasized because attempts which have heretofore been made to construct a satisfactory double cell microphone of this type have failed because of the failure to employ proper yielding connections between the contact plates of the two cells which would permit an equal and properl yielding such as is afforded by the soft rubber casings.

It will be noted that the electrical circuits required are very simple. No induction coils or transformers are required. The elimination of the inductance of the transformer renders the apparatus more sensitive Jfor a given change in the resistance of the microphone. Also since no inductive devices. such as transformers, are required no condenser is necessary around the switch S to prevent the microphones from packing in opening the circuit. Packing' is a permanent decrease of the resistance of the microphone and which may be caused by the inductive kick when a microphone circuit containing inductance is opened. The elimination ot' inductance also prevents packing if the battery circuit is-accidentally opened at any other point. This is of particular importance in submarine sound detection where the microphones are frequently placed in inac cessible 'positions where they cannot be reached to be mechanically rapped to restore them from a packed condition.

Yhile the preferred embodiment of the invention has been specifically illustrated and described. it is to be understood that the present invention is not limited to its illustrated embodiment but may be embodied in other structures within the scope of the following claim.

I claim: i

An inertia microphone having two cells arranged end to end and having the center plates and end plates thereof respectively rigidly connected together. one of said sets of plates being' connected to be vibrated by the received sound waves and the other set ofl platesacting by inertia to resist such vibrations, and soft rubber casings for the two cells permitting the inertia element to float and `vielding to permit substantially equal relative movements of the plates of each cell under the received vibrations. substantially as described.

PERCY lV. BRIDGMAN. 

